The invention is directed to a device for contacting vapors with solids and particularly to vessels having a stationary foraminous bed of solids as the solids contacting medium.
A particularly important aspect of the broad subject of contacting vapors with solids is the design and operation of packed towers, i.e. vessels, usually vertical cylinders, containing packing materials to assure and to facilitate contact between vaporous feeds to such vessels and solids contained therein. The use of packed towers to effect intimate contact between vapors and solids is quite widespread for a vast number of operations such as non-catalytically reacting gases with solids, transferring heat between vapor and solid phases and, of course, for effecting a wide variety of catalytic reactions of the vapor phase.
All of the above-named operations involve problems of gas phase mass transfer in which (1) the gas molecules must be diffused from the bulk of the fluid phase (here we are dealing only with vapors) to the external geometric solid surface, (2) interact with the surface and then (3) be rediffused from the surface to the bulk fluid phase. The more complicated and also the more common instance of this particular operation is the heterogeneous catalysis of the reaction of vapors, which involves the following mechanism:
A. DIFFUSION OF A VAPOR MOLECULE FROM THE BULK OF THE VAPOR PHASE TO THE CATALYTIC SURFACES OF THE SOLIDS, WHICH FREQUENTLY ARE INTRAPOROUS AS WELL AS EXTERIOR;
B. ADSORPTION ONTO SUCH CATALYTIC SURFACES;
C. REACTION OF THE ADSORBED SPECIES WITH THE CATALYTIC SURFACE OR WITH A PRE-ADSORBED REACTANT SPECIES THEREON;
D. DESORPTION OF THE GAS MOLECULE FROM THE CATAYTIC SURFACE; AND
E. REDIFFUSION FROM THE CATALYTIC SURFACES BACK INTO THE BULK OF THE VAPOR PHASE.
The manner and speed with which these steps take place is controlled by factors such as the fluid dynamics, the properties of the catalyst, difference between the properties of the fluid reactant and the products of the reaction, adsorption and desorption energy requirements, activation energy of the catalytic surface and thermal factors such as temperature and heat transfer characteristics.
In view of the complexity of such systems, the design of fixed bed catalytic reactors has frequently been wasteful in that they were overdesigned and therefore were operated at low efficiencies much of the time, especially under operating conditions in which the mass transfer rate was primarily limiting efficiency, i.e. steps (a) and (e) above.